1. Field of the Invention
This invention relates to security systems, and more particularly, to an intrusion alarm system for securing a transmission line which interconnects a protected area with a monitoring area.
2. Description of The Prior Art
In an intrusion detection system, the line of communication between the protected area and the central monitoring area is the most accessible and the most vulnerable link for attack. The ability of attackers to compromise security techniques commonly used in intrusion detection systems has necessitated the development of more sophisticated techniques to minimize the vulnerability of the communication link.
A fairly high degree of alarm security and integrity has been obtained in some systems by using a DC alarm line monitor. Even though such systems provide a moderately high level of line security, it is still defeatable by a skilled attacker.
Accordingly, more sophisticated line security techniques have been developed. Recent advances include the utilization of signals of a pseudo-random nature as a line security code which is generated continuously, or periodically, and transmitted to a monitor over the communication link. The line security code may be encoded with alarm status information which is decoded and displayed at the monitor. In one such system, disclosed in our U.S. Pat. No. 3,735,353, entitled "Alarm Transmission Line Security System Utilizing Pseudo-random Coding," identical arbitrary binary codes are generated by linear shift register code generators located in both the protected area and the monitoring area and the codes are compared in the monitoring area. The failure of the codes to agree is registered as an alarm. The arbitrary code bits are pseudo-random in nature and intrusion alarms are indicated by systematic complementing of the code bits issuing from the protected area.
While the use of pseudo-random code techniques provides a high degree of security for the transmission line and makes it difficult to compromise the system, synchronization of the two code generators is essential and, in the event the code generators get out of synchronization, the system may be in a virtually continual state of alarm. This might happen, for example, if a noise pulse was accepted as a clock pulse by the code generator at the protected area, or if an attacker deleted or added an extra clock pulse.
A further system which minimizes such synchronization problems employs a code generator at a monitoring area which generates a purely random code sequence which is transmitted over the transmission line to the protected areas and encoded with alarm data. The digital random signal is also encoded at the monitoring area to produce coded signals for comparison with the encoded alarm signal received from the protected area. The received alarm code is compared with the encoded sequence generated at the monitoring area, the alarm status of the protected area being extracted as a result of the comparison.
Thus, two approaches have been indicated above. One of the approaches involves use of a pseudo-random code which is continuously generated. The other approach involves the use of a completely random code. It would be desirable to have a security alarm system characterized by the simplicity of the pseudo-random code generation as well as the randomness of the line codes as provided when a random signal generator is employed. It would further be desirable to be able to arbitrarily modify the random code sequence at the monitoring area whenever modification is desired.
A further consideration is that present security practices require the ability to transmit fundamentally different types of signals in each communication direction. Thus, in addition to the transmission of line security code and alarm data, it is often desirable to transmit to the monitoring area, the status of access/secure switches located in the protected area and to activate from the monitoring area a motion simulator in the secured area to accomplish a remote test function.
One example of a system which enables bidirectional transmission of different information is disclosed in the U.S. Pat. No. 3,792,470 of John C. Donovan, Carl F. Klein, Lawrence B. Korta, and Ramesh Krishnaiyer, which is entitled "Coded Tone Multiplexed Alarm Transmission System." In this system, a tone generator driven by a pseudo-random code generator provides a line supervisory code comprised of a selected sequence of tone signals of first and second frequencies which is transmitted to the central monitor and compared with a reference code. Access/secure status is transmitted to the central monitor by enabling the tone generator to provide tone sequences comprised of different pairs of frequencies for indicating either an access or a secure condition for the protected area. The system further provides for remote test of alarm sensors through the generation of a test command which is transmitted to the protected area for enabling a suitable motion simulator apparatus.
While the referenced system has the ability to transmit different types of signals between the protected area and the monitoring area, it would be desirable to have a system which enables information in addition to access/secure status and line monitoring codes to be transmitted from the secured area to the monitoring area and further commands other than synchronizing signals and test commands to be transmitted from the monitoring area to the protected area over the common transmission line.